<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <form name="form">
      <p>
        <select name="city">
          <option value="nj">南京</option>
          <option value="sh" selected>上海</option>
          <option value="bj">北京</option>
          <option value="sz">苏州</option>
        </select>
      </p>
    </form>
    <script>
      console.log(document.form.city);
      console.log(document.form.city.value);

      setTimeout(() => {
        document.form.city.value = "bj";
      }, 5000);
    </script>
  </body>
</html>
